overwintering large containers on deck

I'm planning an overhaul of my south facing very large deck. I'd like to plant some evergreens on it for year round interest (supplimented with colorful annuals in the summer).

For the evergreens, I will use large containers but how big and what type of plant and container is best? I'm concerned about low temperature damage in the winter.

I'll have to leave the large containers on the deck. How can I protect them there? What can I put over them or around them? I've lived in Tacoma for almost 25 years and I've only seen temps here stay below 25F a few times, and only for a few days. How well do plants in large containers survive these cold snaps, with and without additional protection?

Just to expand on the helpful info you've already received, let me add some local experience :-) I live in Kitsap county (close to the water) and I garden almost exclusively in containers due to the steep geography of my location. Also on a south facing deck (primarily). Anything that you can grow in the ground can be grown in a container, provided you accommodate appropriately for size and growth, so you are really not restricted much by plant selection.

In addition to my collection of dwarf conifers, I also grow about a dozen Japanese maples in containers, various shrubs, bamboos and assorted perennials and grasses. Also a herb garden. None of my containerized plants have suffered significantly from any cold damage - a few leaves nipped is about it. I do use pretty good sized containers which provides a lot of soil mass protection around the root system - the most cold vulnerable part of most hardy plants.

So pick out what you like! Just make sure you size the container appropriately and use a very high quality, textural, durable potting soil.

Since unless the containers are pretty big tubs or planters the plants are going to be subject to a lot more freezing of the soil than they would be growing in the ground you do need to choose hardier types like cold climate conifers.

Organized tests have shown a ~20F differential between hardiness of plants in pots and the same kinds in the ground. In other words, an abelia that freezes to near the soil line at 0F in the ground might be zapped by 20F in an unprotected nursery pot.

As mentioned the main problematic result of freezing of containers is death of roots - tops looking okay after a cold spell does not automatically indicate that the entire plant was unaffected. The sequence of death due to cold exposure in a pot is new roots > old roots > top. Especially with one gallon stock it is easy to find specimens around here with large proportions of dead or absent feeder roots out near the pot walls. In addition to cold exposure many small roots are killed by high temperatures on the south sides of pots during sunny weather.

This one did just fine on my deck this winter. It's all about choosing the right type of plants that can handle the conditions they are growing in. This is a little on the extreme side, but I think it makes a point.
Mike

One other thing to consider is the length of time that your plant is subjected to the cold. Suppose that you have a plant in a container that is hardy (in the ground) to 10 F and you experience an early morning low of 18 F. If the temperature is back up above freezing by 10 or 11 am, the plant's roots might be OK. (Or they may not, as there are many variables involved.) Prolonged cold snaps, where the temperature remains below freezing for most of the time for several days on end, are much more problematic.

If you're planting anything that's marginal in containers in this region, you have two options: (1) Make sure that the container is small/light enough to move into a warmer location during these periods. This is the best option for warmer-climate plants that don't like wet feet in the winter. (2) If they're native and/or tolerate wet winter soil, plant into a very large container, as suggested earlier.